1. Field of the Invention
The present invention relates to a method of carrying out temperature drift correction of a measuring machine (on-board measuring machine) provided on a machine tool and measuring a shape of a workpiece by using the on-board measuring machine and to a machine tool on which the measuring machine for measuring the shape of the workpiece is provided.
2. Description of the Related Art
As shown in FIGS. 1A and 1B, there is a well-known three-dimensional measuring machine for measuring a shape of a measured surface of a measured object by measuring positions in a height direction (i.e., a vertical direction) including components orthogonal to a scanned surface while scanning the measured surface of the measured object with a probe in a two-dimensional manner.
This three-dimensional measuring machine scans the measured surface of the measured object at relatively low speed of several tens of millimeters/second or lower by using the probe and therefore it takes a relatively long time over ten minutes to measure the whole face of the measured surface of the measured object. The three-dimensional measuring machine itself has 1-nanometer or better measurement resolution of a single axis in each axial direction and measurement performance of the overall three-dimensional measuring machine is secured by controlling temperature in an ambient environment of the measured object so that temperature change is 1° C. or smaller.
However, if the measurement takes 10 minutes or longer, the three-dimensional measuring machine itself may suffer from local deformation because of its mechanism due to thermal expansion of members forming the three-dimensional measuring machine. Therefore, even with sufficient design consideration, errors depending on the temperature change in the measurement environment are superimposed on measurement values obtained as a result of the measurement. The error in the measurement value is mainly caused by thermal expansion or thermal contraction of the members forming the three-dimensional measuring machine and is a slow component with a relatively long period synchronized with a time period of the temperature change. Hereafter, the error in the measurement value will be referred to as a temperature drift.
According to the nature of the thermal expansion or the thermal contraction of the members forming the three-dimensional measuring machine, it takes the temperature drift due to the change in the ambient temperature more than several hours to become stable in many cases and the temperature drift tends to increase with time as shown in FIG. 2A.
To cope with this, there is an example of the three-dimensional measuring method and machine, as disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2006-138698, which can correct a temperature drift even if the temperature drift occurs in the measuring machine due to deformation of the measuring machine itself during long-time measurement.
According to the technique disclosed in this Patent Document, a probe scans a measured surface along first scanning lines including a plurality of concentric circles about a center point of the measured surface through which an axisymmetry line of an axisymmetric workpiece (e.g., a lens and a semiconductor wafer) passes and second scanning lines including two straight lines (cross lines) passing through the center point, respectively, to obtain coordinate data (here, scanning along the first scanning lines takes a longer time than scanning along the second scanning lines and, as a result, the three-dimensional measuring machine suffers from drifts along coordinate axes direction due to the change in ambient temperature around the machine). Then, coordinate data at intersecting points of the first scanning lines and the second scanning lines are extracted from the coordinate data and drift amounts in the three-dimensional measuring machine are obtained from the extracted coordinate data. Then, by using the drift amounts, the coordinate data including measurement errors are corrected. If there are not coordinate data at the intersecting points of the first scanning lines and the second scanning lines, the intersecting points are calculated by interpolating shape measurement data obtained by scanning along the first scanning lines.
The above-described technique can be applied to only the axisymmetric workpiece, which can be measured along the cross lines, and cannot be applied to the workpiece not in the axisymmetric shape. Moreover, calculation of interpolation for obtaining the coordinate data at the intersecting points of the first scanning lines and the second scanning lines is carried out based on approximation by using four arithmetic operations and the like and therefore nanoscale deviations from actual intersecting points may occur and the deviations directly result in errors caused by the interpolation.
Furthermore, the above-described technique requires special software for operations of comparison processing between the measurement data along the first scanning lines (concentric circles) and the measurement data along the second scanning lines (cross lines), interpolation processing based on approximation, and correction processing of the drift amount. As a result, an operation amount for the correction processing increases in proportion to the measurement data amount and a processing time may become long. Therefore, a cost of the software and a prolonged takt time for one workpiece increase a cost.
In order to achieve nanometer shape accuracy in ultraprecision machining, a machined shape needs to be measured on an ultraprecision machine (i.e., on-board measurement) without detaching a machined workpiece from the ultraprecision working machine and corrective working needs to be carried out based on a measurement result. However, to achieve such correction, the on-board measuring machine needs to have 1-nanometer or better measurement resolution.
In the ultraprecision working machine, a drive portion and a support portion of the machine are generally disposed in an internal space (hereafter referred to as “an inside of the machine”) of the ultraprecision working machine isolated from an outside of the machine in order to maintain nanometer positioning accuracy. Temperature of the inside of the machine is constantly controlled with high accuracy by using a temperature adjusting machine in order to maintain a constant temperature irrespective of change in outside temperature. Therefore, a 1° C. or smaller change in the temperature outside the machine does not affect the positioning accuracy of the drive portion and the support portion disposed inside of the machine.
On the other hand, the on-board measuring machine is mounted in the same space as a working attachment (e.g., a spindle) in order to measure the workpiece on the spot basically without a change of a setup after the machining. The space is not the internal space of the ultraprecision working machine but a place where a worker can easily approach for the machining setup.
Therefore, if the temperature around the ultraprecision working machine changes slightly, the temperature of the on-board measuring machine changes as well according to it. Among the members forming the on-board measuring machine, a probe mounted with a linear scale is an extremely small part. Therefore, a minute temperature change of about 0.1° C. causes thermal expansion or thermal contraction of the probe and a temperature drift is superimposed on a displacement detected by a position detecting device such as a linear scale. Moreover, depending on material of a case member of the on-board measuring machine to which a laser head for detecting the displacement of the linear scale is attached, thermal expansion might occur to similarly cause a temperature drift.
In a normal machining center, such a minute temperature drift hardly has an influence. However, at least in the ultraprecision working machine required to have 100-nanometer or less shape accuracy, even a minute thermal fluctuation might cause the temperature drift of the on-board measuring machine to reach several tens of nanometers to several hundreds of nanometers, which results in a fatal error.
Therefore, the temperature drift of the on-board measuring machine needs to be corrected by a different means from that used by the ultraprecision working machine. Especially, this correction is more crucial to the three-dimensional measurement for measuring the whole face of the workpiece than to the prior-art measurement by scanning the measured surface along the two straight lines (cross lines) with the probe.
The technique described in the above-mentioned JP-A No. 2006-138698 includes scanning of the measured surface along two straight lines (cross lines) passing through the center point of the measured surface through which the axisymmetry line passes with the probe (hereafter referred to as “cross-line measurement”). Because the cross-line measurement finishes in a short time, an influence of the temperature drift on the measurement accuracy is ignorable. This is because the temperature drift tends to gradually increase with time in general (see FIG. 2A).
However, the above-described cross-line measurement cannot be carried out in on-board measurement of a workpiece not in the axisymmetric shape to which the technique described in the above Patent Document is not be applied and therefore it is necessary to scan the whole face of the measured surface of the workpiece with the three-dimensional measuring machine. Depending on a measured area and a measurement pitch of the workpiece, there are a large number of measuring paths along which the workpiece is scanned as shown in FIG. 1 and required measurement time might exceed hundreds of times that of the cross-line measurement.
Although the temperature drift within the measurement time along one measuring path is small, the respective measuring paths suffer from different temperature drifts in measuring the whole face of the workpiece. Therefore, the on-board measurement carried out for a long time is more susceptible to the temperature drift. The longer the measurement time, the likelier it becomes that the measurement accuracy is impaired. Therefore, it is necessary to correct the temperature drift in order to achieve the nanoscale shape accuracy.